Huge Proxima Centauri flare blasts nearest exoplanet Proxima b

On March 24, 2017, astronomers led by Carnegie’s Meredith MacGregor and Alycia Weinberger discovered that a giant stellar flare erupted from Proxima Centauri, the closest star to the Sun (Proxima Centauri means ‘nearest [star] of Centaurus’), using data from the Atacama Large Millimeter/submillimeter Array (ALMA). The huge flare was 10 times larger than a major solar flare. And it blasted Proxima Centauri b (also called Proxima b), the exoplanet orbiting within the habitable zone of Proxima Centauri, with 4,000 times more radiation than Earth gets from solar flares. The event probably wiped out the exoplanet’s atmosphere (if exists any), and dimmed the last hopes of extraterrestrial life on it. This finding is published in The Astrophysical Journal Letters on February 22, 2018.

An artist’s impression of a superflare from Proxima Centauri, the closest star to the Sun (Proxima Centauri means ‘nearest [star] of Centaurus’), modeled after the loops of glowing hot gas seen in the largest solar flares. The exoplanet Proxima b is shown in the foreground. Proxima b, the closest known exoplanet to the Solar System, orbits its star 20 times closer than the Earth orbits the Sun, at a distance of roughly 0.05 AU (7,500,000 km; 4,600,000 mi). A flare 10 times larger than a major solar flare would blast Proxima b with 4,000 times more radiation than the Earth gets from our Sun’s flares. At its peak, the flare increased Proxima Centauri’s brightness by 1,000 times over 10 seconds. Image credit: Roberto Molar Candanosa, Carnegie Institution for Science / NASA / SDO / JPL.

The discovery of Proxima b was announced on august 24, 2016 by a group of scientists led by Dr. Guillem Anglada-Escude at the Queen Mary University of London. First, it was dubbed as “the Earth’s cousin“, because it is at the right place to have a temperature that allows the presence of liquid water on its surface. Then, researches of France’s National Center for Scientific Research (CNRS) have calculated the size and surface properties of the planet, and concluded it may be an “ocean planet” similar to Earth. The planet was looking like a prime spot to hunt for extraterrestrial life.

But, in the following months, in a study published on The Astrophysical Journal Letters, researchers announced that an Earth-like atmosphere may not survive on Proxima b. Because the host star, Proxima Centauri, has quite high magnetic activity. The study has shown that “an Earth-like planet would not survive the escape of its atmosphere at that location, and therefore the pathway to habitability for Proxima b requires a very different atmospheric history than that of Earth.” With the highest thermosphere temperatures and a completely open magnetic field, Proxima b could lose an amount equal to the entirety of Earth’s atmosphere in 100 million years — that’s just a fraction of Proxima b’s 4 billion years thus far. When the scientists assumed the lowest temperatures and a closed magnetic field, that much mass escapes over 2 billion years.

This new discovery of a superflare put the last nail in the coffin of the hopes of a possible extraterrestrial life on Proxima b. Meredith A. MacGregor of the Carnegie Institution for Science in Washington, D.C. and the lead author of the study said that: “I’s likely that Proxima b was blasted by high energy radiation during this flare”. She also explained that it was already known that Proxima Centauri experienced regular, although smaller, x-ray flares. “Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water.”

The circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. Planets within the habitable zones are naturally the best candidates for potential alien life.

But, red dwarfs like Proxima Cantauri are smaller and cooler than the Sun-like stars, and as a result, their habitable zone is much closer to the host star. That causes some problems as the red dwarfs are more active. They would bombard their tightly-orbiting planets with X-rays and ultraviolet radiation, most probably stripping away any atmosphere they may otherwise have.

Red dwarfs like Proxima Centauri or the TRAPPIST-1 star are often the target of exoplanet hunts, because they are the coolest, smallest and most common stars in the galaxy. Because they are cooler and dimmer, planets have to maintain tight orbits for liquid water to be present. But unless the atmospheric loss is counteracted by some other process, such as a massive amount of volcanic activity or comet bombardment, this close proximity is not promising for an atmosphere’s survival or sustainability.

Sources

Study: “On the Magnetic Protection of the Atmosphere of Proxima Centauri b” by K. Garcia-Sage1, A. Glocer, J. J. Drake, G. Gronoff, and O. Cohen. Published on 2017 July 24 on The American Astronomical Society.